Electrical discharge machining power supply with protective system for output switch failure

ABSTRACT

A protective system for an electrical discharge machining power supply circuit including a periodically operated electronic output switch or bank of output switches and a drive stage therefor. A sensing network is connected between the output stage and the gap. Responsive to failure of one or more of the output switches and short circuiting of the gap, the remainder of output switches are biased off and there is provided an interruption of down-feed signal in the servo feed control circuit causing a back-off of the servo system until the &#39;&#39;&#39;&#39;failed&#39;&#39;&#39;&#39; switch has been replaced in the circuit.

United States Patent 1 Sennowitz ELECTRICAL DISCHARGE MACHINING POWERSUPPLY WITH PROTECTIVE SYSTEM FOR OUTPUT SWITCH FAILURE [75] Inventor:Kurt I-I. Sennowitz, Royal Oak,

Mich.

[73] Assignee: Colt Industries Operating Corp., Davidson, NC.

[22] Filed: Jan. 28, 1972 [21] Appl. No.: 221,738

Related US. Application Data Division of Ser. No. 74,997, Sept. 24,1970, Pat. No. 3,665,144, which is a continuation-in-part of Ser. No.874,574, Nov. 6, 1969, Pat. No. 3,548,142, which is acontinuation-in-part of Ser. No. 699,443, Jan. 22, 1968, Pat. No.3,524,036.

[52] US. Cl. 219/69 S, 219/69 C, 219/69 G [51] Int. Cl. B23p 1/08, B23pH14 [58] Field of Search 219/69 C, 69 S, 69 G Oct. 23, 1973 [56]References Cited UNITED STATES PATENTS 3,665,144 5/1972 Sennowitz 219/69C X Primary Examiner-R. F. Staubly AttrneyRobert C. Hauke et al.

[ 7] ABSTRACT A protective system for an electrical discharge machiningpower supply circuit including a periodically operated electronic outputswitch or bank of output switches and a drive stage therefor. A sensingnetwork is connected between the output stage and the gap. Responsive tofailure of one or more of the output switches and short circuiting ofthe gap, the remainder of output switches are biased off and there isprovided an interruption of down-feed signal in the servo feed controlcircuit causing a back-off of the servo system until the failed switchhas been replaced in the circuit.

Claims, 2 Drawing Figures 1 MUL T/ V/BRA TOR PATENIED 00! 23 I8733767386 SHEET 20F 2 '1 ELECTRICAL DISCHARGE MACHINING POWER SUPPLY WITHPROTECTIVE SYSTEM FOR OUTPUT SWITCH FAILURE CROSS REFERENCE TO RELATEDAPPLICATIONS AND PATENTS This application is a divisional application ofmy copending US. Pat. application No. 74,997 for Electrical DischargeMachining Power Supply With Protective System for Output Switch Failure,filed on Sept. 24, 1970, now US. Pat. No. 3,665,144 issued on May 23,1972, which application is a continuation-in-part of my U.S. Pat.application No. 874,574 filed on Nov. 6, 1969 for Gap Short CircuitControl System for Electrical Discharge Machining Apparatus, now issuedas US Pat. No. 3,548,142, which patent in turn is a continuation-in-partof my US. Pat. application No. 699,443 filed Jan. 22, 1968 for ServoControl Circuit for Electrical Discharge Machining Apparatus, now issuedas US. Pat. No. 3,524,036.

BACKGROUND OF THE INVENTION I vided through an electronic pulsegenerator in which an electronic output switch, such as a semiconductorswitching device, is alternately triggered to turn it on and off andthusperiodically connect a power source to the gap to cause passage ofmachining power pulses across the machining gap. An electrode orworkpiece servo feed is employed to maintain an optimum gap spacingbetween electrode and workpiece as metal removal progresses. Adielectric coolant, such as kerosene or transformer oil or the like, iscirculated continuously through the gap during machining operation. Aservo control circuit is required which is capable of providing arelatively constant gap under both roughing and finishing conditions asthe material is being removed from the workpiece. Backup and down-feedare provided at appropriate times to maintain an optimum gap spacing.Related to that problem is the situation arising from gap short circuitcondition when the output switch or one of the switches in the outputswitch bank fails with its principal electrodes in a shorted condition.This results in a continuous discharge across the gap. This continuousdischarge does not permit transfer from point to point on the workpieceas is required for normal machining. The switch short circuiting can bepartial or complete'and can arise from internal damage, thermal runaway,high voltage transients or other circuit malfunctions. Prolongation ofthis condition can cause damage to both electrode and workpiece. l havefound that this condition can best be alleviated by causing servo backupand, optionally, at the same time, bi-

' asing off the remainder of the outputswitches so long as the failedoutput switch or switches remain in the circuit. s

SUMMARY OF THE INVENTION least two principal or power electrodes actingto control current flow in the power circuit, the conductivity betweenthe principal electrodes being controlled by a control electrode withinthe switch whereby the conductivity of the power circuit is controlledstatically or electrically without movement of mechanical elementswithin the switch. Included in this definition are electronic tubes,transistors and like devices. The output switches are biased off" uponfailure of one or more of their number. 'At the same time, a normaldown-feed signal from the gap is inhibited and a backup bias is used toinitiate an electrode or workpiece withdrawal from the gap, whichwithdrawal is maintained until the failed switch is removed andreplaced.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a combined schematic anddiagrammatic showing of my invention; and

FIG. 2 is a schematic showing with a modification of the servo controlportion of that circuit.

DETAILED DESCRIPTION The electrical discharge machining power supply ofFIG. 1 includes a main machining power source 10. Machining power pulsesare passed to a gap between a tool electrode 12 and a workpiece 14through periodic operation of an output transistor 16 which has itspower electrodes, namely, its collector and emitter, in series betweenthe power source 10 and the gap. A number of transistors 16 may beconnected in parallel to provide an output bank of the required currentconducting capability. The manner in which additional transistors areconnected is shown in dash line representation. The output transistor 16is off in the absence of triggering pulses being supplied to its base.The off bias is provided to the base of transistor 16 from a separatebias source 17. Bias voltage is adjustable through series rheostat 19. Afixed resistor l8 and a rheostat 20 are series connected between theemitter of the transistor 16 and the gap to provide for selectivecontrol of cutting current being passed to the gap. A pulse generator,embodied as a multivibrator 22, is used to provide selectively variableon-off time, variable frequency triggering pulses, i.e., drive signals,to the base of a drive stage transistor 24, which in turn operates theoutput transistor 16. Several types of multivibrators suitable for usein this type of electrical discharge machining power supply are shownand described in Lobur US. Pat. No. 3,243,567 issued on Mar. 29, 1966and entitled Electrical Discharge Machining Apparatus. While myinvention is shown in connection with a transistorized EDM power supply,it is not so limited, but may be employed in any supply where anelectronic switch, as I have defined ithereinabove, is connectedbetweeen a DC power source and the machining gap and periodicallytriggered into operation.

My servo control circuit has its reference voltage derived from aresistor network connected in shunt with normally biased by thepotentiometer 30 to provide down-feed and backup of the servo feedmeans. Drive signal charges a capacitor 34 through a resistor 36 and adiode 38, which diode is poled as shown. The diode 38 is further shuntedby a resistor 40. The connection of the drive signal sensing network iscompleted to the gap through a diode 42 whereby drive signal may bepassed into the gap under conditions of output switch failure as will beexplained hereinafter.

The direction of electrode movement is controlled by the direction ofcurrent flow through an electrically energized control element, in thiscase an electromagnetic servo valve coil 44 connected beteen therespective collectors of a transistor 46 and a transistor 48. The fulldetail of the hydraulic motor and servo valve used are not shown hereinin the interest of simplifying this disclosure. However, a number ofsuitable electro hydraulic EDM servo control systems are shown anddescribed in Webb U.S. Pat. No. 3,230,412, issued on Jan. 18, 1966,entitled Servo Feed Apparatus for Electrical Discharge Machining.

A pair of diodes 50 are connected across the baseemitter junction of thetransistor 46 to protect it from excess turn-off voltage. Similarly, apair of diodes 52 are connected across the base-emitter junction of thetransistor 48. A third transistor 49 is coupled to the transistor 46 inemitter-follower arrangement with a protective diode 47 included. A pairof resistors 50 and 51 are connected in series with theemitter-collector of the transistor 49, with the magnitude of theresistor 50 being substantially lower than that of the resistor 51. Theinclusion in the circuit of the transistor 49 makes possible the use ofahigh impedance input to the servo control circuit. During low currentcutting operation, stable servo operation is assured. A rheostat 54 hasits variable resistor connected between the collector of the transistor48 and the right-hand terminal of the coil 44 to provide for adjustmentfor sensitivity of the servo operation. A parallel RC network isconnectedin series circuit between a fixed resistor 55 and the collectorof transistor 46. Included in the RC network are a resistor 64, aresistor 65 and a capacitor 66. A fast backup diode 67 is connected withthe polarity shown. The resistor 65 is included in order to limit thedischarge of the capacitor 66. A pair of current limiting resistors 74and 76 are connected in circuit with the respective bases of thetransistors 46 and 48. A load resistor 78 and a load resistor 80 areconnected between the respective collectors of transistors 46 and 48 andthe negative voltage terminal of the DC source 10, while a singleresistor 82 is coupled between the positive terminal of the DC sourceand the emitters of the transistors 46, 48.

The servo control circuit of FIG. 2, similar to that of FIG. 1, includesa reference voltage source which is derived from a main machining powersource 110 by means of a shunt resistor network including a pair offixed resistors 130, 132 and the variable resistance of potentiometer134. A filter capacitor 136 is connected as shown. The control elementin the circuit is an electro-magnetic servo valve control coil 138. Thedirection of electrode or of workpiece movement is controlled by thedirection of current flow through the coil 138. The full detail of theassociated hydraulic motor and servo valve are not shown herein in theinterest of simplifying this disclosure. However, a number of suitableelectro hydraulic EDM servo controlsystems may be used as shown in thepreviously mentioned Webb U.S. Pat. No. 3,230,412. A potentiometer iscoupled as shown in parallel with the coil 138 to provide a velocitycontrol for the servo feed. The coil 138 has its right-hand terminaloperatively connected to the gap through a parallel resistor-capacitornetwork, including a resistor 156 and a capacitor 158 with resistor 156being of a relatively high magnitude. The parallel resistor-capacitornetwork is coupledto the gap to sense gap conditions, which in turnprovides an indication of whether the output transistor 16 is operatingnormally to pass machining power pulses across the gap. A resistor 160and a rectifier 162 are connected across the resistor-capacitor networkandthe gap as shown.

DESCRIPTION OF OPERATION With reference to FIG. 1, when the power supply10 and the multivibrator 22 are turned on, drive signals are passed fromthe transistor 24 to control the operation of the output transistor 16.Machining power pulses are provided across the gap with preciselycontrollable frequency and on-off time factor. At the same time, thenegative drive signal is passed through the resistor 36 and the diode 38to be stored on the upper plate of the capacitor 34. A portion of thisdrive signal will be passed to the gap through the diode 42. Thenegative drive signal stored on the capacitor 34 provides turn-on of thetransistor 49 and subsequent turnon of the transistor 46. Electron flowthen occurs from the negative terminal of DC source 10 through theresitor78, the collector-emitter of the transistor 46 and throughresistor 82 to the positive terminal of the DC source 10. At the sametime, electron flow occurs through the resistor 80, rheostat 54, in aleftward or down-feed direction through the coil 44, through theresistors 55, 64 and through the collector-emitter of the transistor 46.This electron flow in a leftward direction through coil 44 causes theelectrode 12 to be advanced downwardly through the work 14. During ashorted gap, the other transistor 48 will be turned on providing interalia an electron flow through the coil 44 in a rightward or up-feeddirection to withdraw the electrode 12 and move it upwardly away fromthe workpiece 14 to permit the short circuit condition to be cleared. Itis further significant that during a short circuit condition withexcessive current flow through the gap whether the condition is causedby output transistor 16 failure or by eroded particles bridging the gap,the electrode 12 is being withdrawn from the workpiece. At the sametime, by reason of the sensing network used including the resistor 36,the diode 42 and its connection to the gap, the drive signal is beingvirtually all passed to the positive gap terminal or ground. In theevent a bank of output transistors 16 are used, bias source 17 will holdoff the remainder of the transistors until the 'back-off occurs and thedefective transistor has been replaced. This mode of operation furnishesboth prompt and effective backup and gap current limiting at the sametime.

An additional feature is that of the incorporation of the rheostat 54network in series with the control coil 44. This is important for stableservo action during high frequency cutting. During normal cutting, thehydraulic control coil 44 will pass current in one direction only andhold the gap constant according to the reference voltage settingpreviously made on the potentiometer 30. In the circuit of FIG. 1, ifthe gap should short circuit, the capacitor 66 will discharge rapidlyand provide fast ram backup, cutting away particles which may have beenaccumulated in the gap and in breaking up the short circuit condition.This provides highly effective pulsing of the gap during up-feed ordown-feed and particularly improves deep hole cutting by clearing theshorted gap more quickly, thus preventing the electrode from backing outall the way.

The circuit of FIG. 2 is one which incorporates a servo back-offresponsive to failure of the gap output transistor 16 or responsive tofailure of the intermediate stage drive transistor 24. Following thefailure of an output transistor 16, a number of related events takeplace. When transistor 16 fails in a shorted condition, this causes thegap to experience an abrupt rise in passage of current thereacross. Biaswill be applied to the control electrodes of the other transistors 16 tohold them of The sensing network comprising RC network 156-158 as hasbeen mentioned incorporates a very large magnitude resistor 156 so thatthe capacitor 158 can only pass pulsating DC during the normal operationof the output transistor. Once there has been a failure of the outputtransistor 16, there will be no downfeed signal available to controlcoil 138. The reference signal, as preset by potentiometer 134, willcontinue to pass a backup voltage through control coil 138 to backup theelectrode or the workpiece as the case may be, one from the other, untilthe defective transistor 16 whether it be a single transistor or one ofa bank of several transistors has been replaced.

it will thus be seen that l have provided an improved protective systemfor EDM in which responsive to failure of an output transistor therefollows a hold-off of the other transistors in the output bank if thereare more than one. My system 'at the same time and responsive to thesame conditions provides backup of the electrode to eliminate anypossible further damage to the gap elements through excessive currentflow to the gap, and at the same time gives the operator a visualindication that a transistor failure has occurred.

1 claim:

1. An electrical discharge machining apparatus having a bank of outputelectronic switches periodically operated by a drive signal switch forproviding machining power pulses from a power source across a machininggap between an electrode and a workpiece, wherein a system is providedfor electrode backup responsive to failure of at least one of saidoutput switches, said system including a motive means and a servo feedcircuit operatively connected to and controlling said motive means forproviding a down-feed signal thereto during normal machining conditions,said servo feed circuit further operably connected to said outputswitches and operable responsive to their failure to interrupt saiddown-feed signal to said motive means and thereby increase the spacingof said gap for the duration of such failure.

2. In an electrical discharge machining apparatus having an outputswitch periodically operated by a drive signal switch for providingmachining power pulses from a power source across a machining gapbetween a tool electrode and a workpiece, wherein the improvementcomprises a backup system for providing increased spacing between saidelectrode and the workpiece responsive to failure of said drive signalswitch, comprising a motive means and a servo feed circuit operativelyconnected to and controlling said motive means by providing a down-feedsignal thereto during normal machining operation, said servo feedcircuit operatively connected to the gap and to the drive signal switchto interrupt said down-feed signal to the motive means responsive tofailure of said drive signal switch and thereby to increase the spacingof said gap.

3. In an electrical discharge machining apparatus having an outputelectronic switch periodically operated by a drive signal switch forproviding machining power pulses from a power source across a machininggap between an electrode and a workpiece, wherein the improvementcomprises a system for increasing the gap spacing to the top of thestroke responsive to failure of said output electronic switch, saidsystem including a motive means and a servo feed circuit operativelyconnected to and controlling said motive means by providing a down-feedvoltage signal thereto during normal machining operation, said servofeed circuit further operatively connected to the output switch andoperable responsive to its failure to change the down-feed voltagesignal to a voltage signal of backup polarity.

4. The combination as set forth in claim 3 wherein said motive meansincludes a reversible electro hydraulic motor and an electrical controlmeans operatively connected and controlling the operation of said motor,said control means having a control voltage connectible to one of itsterminals representative of failure of one of said output switches toprovide increased spacing for the duration of such failure.

5. The combination as set forth in claim 3 wherein said motive meansincludes an electrically operated control means, said control meanshaving one of its terminals connected to a reference voltage and theother of its terminals operatively coupled to said drive signal switchoutput for receiving a suitable voltage signal representative of failureof one of said output switches and for thereby increasing the spacing asbetween said tool electrode and workpiece.

6. In an electrical discharge machining apparatus having a plurality ofparallel connected output electronic switches periodically turned on bya drive signal switch for providing machining power pulses from a powersource across a machining gap between a tool electrode and a workpieceand further having a holdoff bias connected to the output switches forholding them off responsive to failure of one of their number, theimprovement comprising a system for providing backup as between saidtool electrode and workpiece subsequent to failure of one of said outputswitches, said system including a motive means and a servo feed circuitoperable to provide a down-feed signal to said motive means duringnormal machining conditions, said servo feed circuit further operablyconnected to and operable responsive to failure of one of said outputswitches to interrupt the down-feed signal to said motive means andthereby increase the spacing of said gap, said motive means being of theelectrohydraulic type and including a servo control coil and anoperational amplifier operably connected to and controlling said coil,said operational amplifier having its one terminal connected to areference voltage having a magnitude variable responsive to both gapvoltage and to the failure of any one of said output switches.

7. In an electrical discharge machining apparatus having a plurality ofparallel connected output electronic switches periodically turned on bya drive signal switch for providing machining power pulses from a powersource across the machining gap between the tool electrode andworkpiece, the improvement comprising a system for providing continuedbackup as between said tool electrode and workpiece subsequent tofailure of at least one of said output switches, said system including amotive means and a sensing network coupled to one terminal of said gapfor responding to pulsating DC voltage level, said servo feed furtheroperable to provide'a down-feed signal to said motive means duringnormal machining condition, said servo feed circuit coupled to saidnetwork and responsive to said DC voltage level resulting from failureof said output switch to interrupt down-feed to said motive means and toprovide continued backup.

8. The combination as set forth in claim 7 wherein said sensing networkcomprises a rectifier and a parallel RC network combination, saidcombination coupled across said gap.

9. In an electrical discharge machining apparatus having a bank ofoutput electronic switches periodi- '8 cally operated by a drive signalsource for providing machining power pulses from the power source acrossa machining gap between a tool electrode and a workpiece, wherein asystem is provided for continuous electrode backup responsive to failureof at least one of said output switches, said system including a motivemeans and a servo feed circuit operatively connected to and controllingsaid motive means for providing a down-feed signal thereto during normalmachining conditions, said servo feed circuit further coupled to saidoutput switches through a sensing network and operable responsive tovoltage drop resulting from such failure to interrupt said normaldown-feed signal to said motive means and therefore increase the gapspacing for the duration of such failure.

10. The combination as set forth in claim 9 wherein said networkcomprises a unilateral current conducting device connected in seriescombination with a parallel RC network, said combination coupled acrosssaid machining gap.

1. An electrical discharge machining apparatus having a bank of outputelectronic switches periodically operated by a drive signal switch forproviding machining power pulses from a power source across a machininggap between an electrode and a workpiece, wherein a system is providedfor electrode backup responsive to failure of at least one of saidoutput switches, said system including a motive means and a servo feedcircuit operatively connected to and controlling said motive means forproviding a down-feed signal thereto during normal machining conditions,said servo feed circuit further operably connected to said outputswitches and operable responsive to their failure to interrupt saiddown-feed signal to said motive means and thereby increase the spacingof said gap for the duration of such failure.
 2. In an electricaldischarge machining apparatus having an output switch periodicallyoperated by a drive signal switch for providing machining power pulsesfrom a power source across a machining gap between a tool electrode anda workpiece, wherein the improvement comprises a backup system forproviding increased spacing between said electrode and the workpieceresponsive to failure of said drive signal switch, comprising a motivemeans and a servo feed circuit operatively connected to and controllingsaid motive means by providing a down-feed signal thereto during normalmachining operation, said servo feed circuit operatively connected tothe gap and to the drive signal switch to interrupt said down-feedsignal to the motive means responsive to failure of said drive signalswitch and thereby to increase the spacing of said gap.
 3. In anelectrical discharge machining apparatus having an output electronicswitch periodically operated by a drive signal switch for providingmachining power pulses from a power source across a machining gapbetween an electrode and a workpiece, wherein the improvement comprisesa system for increasing the gap spacing to the top of the strokeresponsive to failure of said output electronic switch, said systemincluding a motive means and a servo feed circuit operatively connectedto and controlling said motive means by providing a down-feed voltagesignal thereto during normal machining operation, said servo feedcircuit further operatively connected to the output switch and operableresponsive to its failure to change the down-feed voltage signal to avoltage signal of backup polarity.
 4. The combination as set forth inclaim 3 wherein said motive means includes a reversible electrohydraulic motor and an electrical control means operatively connectedand controlling the operation of said motor, said control means having acontrol voltage connectible to one of its tErminals representative offailure of one of said output switches to provide increased spacing forthe duration of such failure.
 5. The combination as set forth in claim 3wherein said motive means includes an electrically operated controlmeans, said control means having one of its terminals connected to areference voltage and the other of its terminals operatively coupled tosaid drive signal switch output for receiving a suitable voltage signalrepresentative of failure of one of said output switches and for therebyincreasing the spacing as between said tool electrode and workpiece. 6.In an electrical discharge machining apparatus having a plurality ofparallel connected output electronic switches periodically turned on bya drive signal switch for providing machining power pulses from a powersource across a machining gap between a tool electrode and a workpieceand further having a hold-off bias connected to the output switches forholding them off responsive to failure of one of their number, theimprovement comprising a system for providing backup as between saidtool electrode and workpiece subsequent to failure of one of said outputswitches, said system including a motive means and a servo feed circuitoperable to provide a down-feed signal to said motive means duringnormal machining conditions, said servo feed circuit further operablyconnected to and operable responsive to failure of one of said outputswitches to interrupt the down-feed signal to said motive means andthereby increase the spacing of said gap, said motive means being of theelectrohydraulic type and including a servo control coil and anoperational amplifier operably connected to and controlling said coil,said operational amplifier having its one terminal connected to areference voltage having a magnitude variable responsive to both gapvoltage and to the failure of any one of said output switches.
 7. In anelectrical discharge machining apparatus having a plurality of parallelconnected output electronic switches periodically turned on by a drivesignal switch for providing machining power pulses from a power sourceacross the machining gap between the tool electrode and workpiece, theimprovement comprising a system for providing continued backup asbetween said tool electrode and workpiece subsequent to failure of atleast one of said output switches, said system including a motive meansand a sensing network coupled to one terminal of said gap for respondingto pulsating DC voltage level, said servo feed further operable toprovide a down-feed signal to said motive means during normal machiningcondition, said servo feed circuit coupled to said network andresponsive to said DC voltage level resulting from failure of saidoutput switch to interrupt down-feed to said motive means and to providecontinued backup.
 8. The combination as set forth in claim 7 whereinsaid sensing network comprises a rectifier and a parallel RC networkcombination, said combination coupled across said gap.
 9. In anelectrical discharge machining apparatus having a bank of outputelectronic switches periodically operated by a drive signal source forproviding machining power pulses from the power source across amachining gap between a tool electrode and a workpiece, wherein a systemis provided for continuous electrode backup responsive to failure of atleast one of said output switches, said system including a motive meansand a servo feed circuit operatively connected to and controlling saidmotive means for providing a down-feed signal thereto during normalmachining conditions, said servo feed circuit further coupled to saidoutput switches through a sensing network and operable responsive tovoltage drop resulting from such failure to interrupt said normaldown-feed signal to said motive means and therefore increase the gapspacing for the duration of such failure.
 10. The combination as setforth in claim 9 wherein said network comprises a unilateral currentconducting device connEcted in series combination with a parallel RCnetwork, said combination coupled across said machining gap.